* Adding ranker similar to retriever and reader
* Sort documents according to query-document similarity scores
* Reranking and model training runs for small example
* Added EvalRanker node
* Calculate recall@k in EvalRetriever and EvalRanker nodes
* Renaming EvalRetriever to EvalDocuments and EvalReader to EvalAnswers
* Added mean reciprocal rank as metric for EvalDocuments
* Fix bug that appeared when ranking documents with same score
* Remove commented code for unimplmented eval() of Ranker node
* Add documentation of k parameter in EvalDocuments
* Add Ranker docu and renaming top_k param
